Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1999-05-04
2001-02-13
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S309000, C324S322000
Reexamination Certificate
active
06188220
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the invention is magnetic resonance imaging, and in particular the invention relates to sensing variation of the B
0
polarizing magnetic field resulting from vibration of the imaging apparatus.
Any nucleus, which possesses a magnetic moment, attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency, known as the Larmor frequency, which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant &ggr; of the nucleus). Nuclei which exhibit this phenomena are referred to herein as “spins”.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B
0
), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M
z
is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. However, if the substance or tissue is subjected to a magnetic field (excitation field B
1
) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M
t
, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B
1
is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance (“NMR”) phenomena is exploited.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G
x
, G
y
and G
z
) which have the same direction as the polarizing field B
0
, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
MRI is particularly useful as a medical diagnostic tool. However, the ability to create detailed images which clearly depict anatomical features of the patient, depends upon an extremely stable polarizing magnetic field B
0
. Mechanical vibration of the MRI system perturbs the polarizing magnetic field, thereby producing artifacts in the resultant magnetic resonance images. If the real-time displacement of the magnetic components could be accurately measured, then mathematical models could be used to estimate and correct the magnetic field variation due to that displacement. For whole body magnets, displacements on the order of a micron generate magnetic field changes of approximately one part per million. Thus a motion sensor for artifact correction must be capable of detecting submicron displacements.
The obvious approach to measuring the vibration would be to sense the variation of the polarizing magnetic field. However, the imaging system produces other magnetic fields, which vary at radio frequencies and thus can adversely affect the ability to sense changes in the polarizing magnetic field.
SUMMARY OF THE INVENTION
The present invention provides technique for detecting minute vibration of a magnet of a magnetic resonance imaging system which results in perturbation of the magnetic field. That technique generates a first signal having a predefined frequency X which is directed toward part of the magnetic resonance imaging system which is physically connected to the magnet. The first signal may be selected from among the microwave, ultrasound, sound and light spectra, for example. The signal also may originate from a resonant radio frequency coil whose properties change with displacement.
A portion of the first signal gets reflected by that part and then is received as a second signal. The first and second signals are processed to produce a output signal which represents movement of the electromagnet. That output signal can be employed in compensating for effects that the vibration has on the magnetic field and ultimately in images produced by the magnetic resonance imaging system.
In the preferred embodiment, the first and second signals are mixed together to produce an resultant signal that then is low-pass filtered. The base-band signal produced by the filtering than is applied to a quadrature detector which produces the output signal representing movement of the electromagnet.
REFERENCES:
patent: 4712560 (1987-12-01), Schaefer et al.
patent: 5427102 (1995-06-01), Shimode et al.
patent: 5652514 (1997-07-01), Zhang et al.
patent: 6008887 (1999-12-01), Klein et al.
Arana Louis
Cabou Christian G.
General Electric Company
Price Phyllis Y.
Quarles & Brady
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